Some traditional internal combustion engines have two parallel fuel injection modes, i.e., a cylinder injection mode, also called “direct injection mode”, and a port injection mode. In other words, the engines use either one or both of cylinder injection valves for injecting fuel into cylinders and port injection valves for injecting fuel into intake ports of the cylinders depending on the operational states of the engines. Various techniques have been suggested for such engines to select a fuel injection mode depending on the loads on the engines and control the timings of injection of fuel.
The fuel injected from the port injection valves partially adheres to the surfaces of intake valves and the walls of the intake ports in the form of liquid layers. The liquidly-layered fuel gradually evaporates depending on the temperatures and pressures of the intake ports and slowly enters the cylinders. Unfortunately, the vaporization of the adhering fuel may take a long time at low temperatures of the intake ports, for example, as in the cold start of the engines. This phenomenon reduces the volumes of fuel introduced into the cylinders, resulting in leaner air-fuel ratios than intended.
A technique suggested to solve this problem is a combination of estimation of the volume of fuel adhering on the wall of an intake port and determination of the volume of fuel injection based on the estimated volume. For example, the technique involves the calculation of the volume of the fuel adhering on the intake-port wall on the basis of the load on the engine, and the correction of the volumes of port injection and cylinder injection on the basis of the calculated volume. An increase in the port injected volume on the basis of the volume of the fuel adhering on the wall leads to a proper ratio of the port injection to the cylinder injection and an optimum air-fuel ratio. If it is advisable to correct the volume of the fuel injected from a port injection valve to a value exceeding the maximum volume, the cylinder injected volume may also be increased to optimize the air-fuel ratio (e.g., refer to Japanese Patent No. 4449706).
In general, the cylinder injection receives fuel injected at a higher pressure than that in the port injection. The fuel is thus readily atomized in the cylinder and hardly adheres on the wall of the cylinder and the top surface of a piston. Unfortunately, the fuel injected from the cylinder injection valve may partially adhere on the inner surface of a combustion chamber in the cylinder in the form of a liquid layer. It is thus difficult to appropriately control the air-fuel ratio without consideration of the effects of fuel adhering on both the port and the cylinder.
In specific, a typical engine having two parallel fuel injection modes, i.e., cylinder injection and port injection modes, selects any one or a combination of these fuel injection modes depending on the operational state of the engine. Accordingly, in a transitional operational state occurring on the switching of fuel injection modes (e.g., immediately after the switching from the cylinder injection to the port injection), the required volume of cylinder injection may fall below the volume of fuel evaporated from the cylinder. In this case, the difference of the required volume of cylinder injection from the volume of fuel evaporated from the cylinder is subtracted from the port injected volume to prevent the state (rich state) of the cylinder containing excess fuel in the current operational state of the engine.
The cylinder injection is more responsive than the port injection and can lead to ready control of the air-fuel ratio. Unfortunately, the adhesion of fuel on the cylinder varies the fuel level in the cylinder and may cancel the advantage of the cylinder injection. For example, in the operational state of the engine that requires the precise control of the air-fuel ratio, the fuel adhering on the cylinder may impair the proper response of the air-fuel ratio. The air-fuel ratio is thus controlled in view of the effects of the fuel adhering on the cylinder to improve the response and the control of the air-fuel ratio.